1. Field of the Invention
The field of the invention is proximity sensors. More specifically the field of the invention is those sensors which utilize electronic oscillation circuitry.
2. Brief Description of the Prior Art
Various proximity sensors utilizing electronic oscillation circuitry appear in the prior art. Hardin U.S. Pat. No. 3,473,110 discloses one such circuit utilizing an LC resonator. The presence of an electrically conducting object in proximity with the probe of this invention causes a decrease in the gain of the oscillation. This decrease in gain is detected by a bi-stable demodulator. In Buck U.S. Pat. No. 3,747,012 the decrease in amplitude of oscillation triggers a thyristor which changes the current flow of the circuit. This causes a signal to be given indicating that an electrically conducting object is in proximity with the probe. In Scruggs U.S. Pat. No. 3,919,629 the oscillation of the resonating circuit ceases because of increased inductance of the probe that is the result of the existence of a metal object in proximity with the probe. Scruggs uses a power switching network that includes a diode bridge and a thyristor.
Prior art circuits generally are not substantially responsive to the induced resistance of the metal object being detected. The circuits typically respond to a change in frequency when a metal object comes into proximity with the probe. The circuits do not utilize a constant current when in operation, tending to make detection of output sensor signals less stable, less uniform in response, or more complicated. Typically the detection circuit in the prior art is not sophisticated, not providing for probe fault indication signals, and not providing for crack detection.